Process for the electrochemical decomposition of powders and electrolysis cells suitable therefor

ABSTRACT

Process for the electrochemical decomposition of precursors in powder form by introducing a powder batch between two electrodes of an electrolysis cell, electrodes being designed to be liquid-permeable, and the electrolyte flowing through the powder batch perpendicularly to the electrode surfaces, and electrolysis cell suitable therefor, which is essentially characterized in that at least one electrode has a structure which consists of a supporting pierced plate ( 5 ), an electrode plate ( 3 ) provided with perforations, and a filter cloth ( 4 ) arranged between the supporting pierced plate ( 5 ) and the electrode plate ( 3 ), and in that the cathode ( 6 ) is shielded from the cell by means of a liquid-permeable separator ( 7 ).

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to a process for the electrolyticdecomposition of precursors in powder form, and in particularhigh-melting alloys such as rhenium/tungsten alloys, rhenium/molybdenumalloys, rhenium/molybdenum/tungsten alloys, superalloys, and alloysystems containing noble metals. More particularly, the inventionrelates to a process of recovering the valuable materials, such asrhenium, tantalum, hafnium and platinum contained in the precursors.Such precursors are encountered when processing the correspondingalloys, and also when comminuting scrap from used machine parts made ofsuch alloys.

2. Brief Description of the Prior Art

For the recovery of valuable metals from alloy scrap, a range ofoxidative metallurgical and hydrometallurgical processes have beendeveloped, but all of them have predominantly the disadvantage of a highenergy consumption (for example roasting at high temperatures) and/orrequire the use of environmentally unfriendly reagents.

A good summary of the various processes is provided by the publicationsKenworthy et al., “Experimental Extraction of Strategic Components . . .”, Report of Investigations 5786, United States Department of theInterior, Bureau of Mines, 1976, and K. Vadasdi, “Effluent FreeManufacture of Ammonium Paratungstate (APT) by Recycling theByproducts”, Int. J. of Refractory Metals & Hard Materials 13 (1995)45-59.

Also mentioned in these publications are, electrolytic processes for thedecomposition of alloys For energy-related and ecological reasons,electrolytic processes are generally preferable to the other processes.The advantages of the electrolytic decomposition processes for scrap aregenerally due to the elegant and streamlined process control, which,with current efficiencies of around 100%, ensures low process costs andalso constitutes a better choice in environmental terms.

The major disadvantage of the electrolysis processes becomes apparentwhen it is necessary to decompose scrap which is present in powder form,typically with particle sizes smaller than 1000 μm, with averageparticle sizes of from 10 to 500 μm and preferably up to 200 μm, withthe particle sizes of as little as 1 μm being included. In addition tothe fundamental problem of handling such powders in an electrolysiscell, bringing a powder particle into contact with the anode, otherproblems arise, in particular, that of susceptibility of such powders tosurface passivation, which leads to almost complete termination of theanodic oxidation processes. Consequently, within short electrolysistimes, an uneconomically high increase in the electrolysis voltage takesplace, together with the breakdown of water. Although for theelectrolysis of scrap in large pieces, it is possible to counteract suchpassivation effects by suitable measures involving the electrolytecomposition, pH adjustment, and current polarity reversal (J.Electrochem. Soc. India, 1986, 35-2, 127), in the case of powderbatches, however, boundary-layer effects with pronounced pH gradientsowing to low electrolyte mobility have a dominant effect and terminatethe intended reaction.

It was an object of the present invention, while taking economic aspectsinto account, to provide an electrolytic process for the decompositionof powders and an electrolysis cell which is suitable for theelectrolysis of powders.

SUMMARY OF THE INVENTION

The invention relates to a process for the electrochemical decompositionof precursors in powder form by introducing a powder batch between twoelectrodes of an electrolysis cell, which is characterized in that theelectrodes are designed to be liquid-permeable, and the electrolyteflows through the powder batch perpendicularly to the electrodesurfaces.

Advantageously, the electrolysis cell has, at least on the outflow sideof the brine, an electrode structure which comprises, in the flowdirection of the brine, an electrode plate provided with perforations, aliquid-permeable, preferably woven filter cloth, and a supportingpierced plate intended to support the filter cloth against the anodeplate.

Advantageously, in order to ensure electrical contact with the anode,the powder batch lies or bears on the anode plate. In order to avoid ashort-circuit, the cathode plate is insulated from the powder batch by aseparator. A suitable separator is a fabric which is resistant to theelectrolyte, for example a PTFE-based (polytetrafluoroethylene-based)fabric, or a perforated plate or sheet of electrolyte-resistantmaterial.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 shows an electrolysis cell 1 according to the invention, whichconsists of an electrically insulating cell housing 2 in which ahorizontally arranged anode 3 provided with perforations is arranged. Afilter cloth 4 bears against the anode 3, and is held against the anode3 by a means of pierced support plate 5. The cathode 6, which has aseparator 7 on its side facing the anode 3, is arranged verticallymobile inside the cell housing 2.

DETAILED DESCRIPTION OF THE INVENTION

The invention is further described hereunder with particular referenceto its present embodiments. According to one embodiment of theinvention, the electrolyte flows through the electrolysis cell from thecathode to the anode, with the powder batch being pressed against theanode not only by gravity, but also by the liquid pressure which isnecessary owing to the flow resistance.

According to another embodiment of the invention, the electrodes arearranged horizontally, with the cathode plate being fitted so as to bevertically mobile in the cell, and lying on the powder batch while beingseparated from it by the separator. In this case, the powder batch may,in order to ensure electrical contact with the anode plate and betweenthe powder particles of the batch, exert pressure on the batch owing tothe weight of the electrode plate or owing to additional forces exertedon the electrode plate, with a short-circuit being prevented by theseparator arranged on the cell side of the cathode.

In the event that the cell is designed with a vertically mobile cathode,the electrode spacing is determined by the height of the powder batch,which decreases in the course of carrying out the electrochemicaldecomposition. An initial height of the powder batch of from 1 to 50 cmis preferred.

When the electrode spacing has fallen to less than 10% of the initialspacing, the cell is opened and again filled with a powder batch.

According to another embodiment of the invention, the electrolysis cellwith horizontal electrodes has a fixed electrode spacing, with thepowder to be decomposed being periodically or continuously dispersed inthe electrolyte, and being fed into the cell by means of an insulatedtube supply through the perforated electrode on the inflow side. Thisfollows a continuous mode of operation.

According to another embodiment of the invention, the cell withhorizontal electrodes has a vertically mobile electrode, by means ofwhich pressure is exerted on the powder batch, discontinuously, forexample every 10 to 50 hours. The electrode spacing is increased, andrefilling is carried out by means of dispersion in the electrolytethrough the perforated electrode on the inflow side, and the verticallymobile electrode is subsequently placed onto the powder batch which hasbeen built up.

Advantageously, the electrolyte flows from top to bottom through theelectrolysis cell with horizontal electrodes. It is likewise preferablefor the lower electrode to be the anode, which has a filter cloth heldby a pierced base on the outflow side.

The electrolyte is selected according to the powder composition to bedecomposed. An aqueous alkali hydroxide solution will advantageously beused as the electrolyte, if the alloy powder comprises metals that formalkali metallates which are soluble in an aqueous alkaline medium inparticular tungsten, molybdenum and/or rhenium and their alloys.Advantageously, an alkali hydroxide concentration of at least 0.1 moleper litre will be maintained. Particularly preferably, the alkalihydroxide concentration should be from 1 to 6 moles per litre. Anaqueous inorganic acid, preferably hydrochloric acid, willadvantageously be used as the electrolyte, in the event that superalloypowders based on the major alloy components nickel, cobalt and/orchromium are used as the powder to be decomposed, in particular thosewhich furthermore contain valuable material components such as Hf, Ta,Nb, Mo, W, Re and/or platinum group metals.

The present invention furthermore relates to an electrolysis cell,comprising an electrically insulating housing and two liquid-permeableelectrodes arranged in the housing, at least one of the electrodeshaving a structure which consists of a supporting pierced plate, anelectrode plate provided with perforations, and a filter cloth arrangedbetween the supporting pierced plate and the electrode plate, and thecathode is shielded from the cell interior by means of aliquid-permeable separator consisting of electrically non-conductivematerial; furthermore, a distribution chamber for the electrolyte isprovided on the cathode side and a collection chamber for theelectrolyte is provided on the anode side, a recirculation line routedoutside the cell is provided between the collection chamber and thedistribution chamber, and means for delivering the electrolyte throughthe recirculation line from the collection chamber to the distributionchamber are provided, and an external heat exchanger, which can beoperated selectively in heating or cooling mode, can be interposed inthe recirculation line.

Advantageously, the electrodes of the electrolysis cell are arrangedhorizontally above one another with a spacing between them, the anodeforming the lower electrode and the cathode forming the upper electrode.It is furthermore preferable for the cathode to be arranged verticallymobile in the housing.

It is furthermore preferable to provide means for removing some of theelectrolysis brine, and for supplying fresh electrolyte, in thedistribution chamber, the collection line and/or the recirculation line.The distribution chamber furthermore has means for drawing offelectrolytically evolved gases, in particular hydrogen.

The invention will be explained in more detail with reference to FIG. 1below: FIG. 1 shows an electrolysis cell 1 according to the invention,which consists of an electrically insulating housing 2 in which ahorizontally arranged anode 3 provided with perforations is arranged. Afilter cloth 4 bears against the anode 3, and it is held against theanode 3 by a means of pierced support plate 5. The cathode 6, which hasa separator 7 on its side facing the anode 3, is arranged verticallymobile inside the cell housing 2. The spacing between the anode 3 andthe cathode 6 is determined by the height of the powder batch 9 betweenthe electrodes. The distribution chamber 10 for the electrolyte islocated above the cathode 6. The collection chamber 11 for theelectrolysis brine is located below the anode 3. The brine is pumpedthrough the recirculation line 12 by means of a schematically indicatedhose pump 13. Optionally, a storage container operated under reducedpressure (vacuum) may be connected upstream, from which the electrolytecan be fed back to the cell, while being regulated according to thefilling level, using a suitable pump. Some of the electrolyte brine iscontinuously or periodically removed via 14 and replaced by freshelectrolyte, as indicated by the arrow 15. Hydrogen 16 evolved at thecathode 6 furthermore escapes from the distribution chamber 10, which isshown here as being open. In another embodiment of the operation of thecell according to the invention, the electrolyte is made to flow upwardsagainst the anode from below, and the electrolyte may thenadvantageously be taken from a free overflow above the cathode of thecell and fed back into the circuit.

The invention is further illustrated but is not intended to be limitedby the following examples in which all parts and percentages are byweight unless otherwise specified.

EXAMPLES Example 1

In electrolysis equipment according to the invention and correspondingto FIG. 1, with a circular base area of 154 cm², 3059 g of Re/W scrap(3% Re, 97% W, average particle size approximately 100 μm) in powderform are poured onto a perforated metal plate (for example Ni) connectedas the anode; the surface of the powder is covered first with aseparator and then with a cathodically connected, likewise perforated,vertically mobile metal plate which is lowered by its own weight. Afilter cloth, which in turn rests on a pierced base for stabilization,is stretched below the anodic metal plate. This basic equipment combinesboth the function of an electrolysis cell and also the function of afilter component, and it will be referred to below overall as afiltration electrolysis cell. The filtration electrolysis cell isprovided with an internal circuit, in which the alkaline electrolyte issucked through the powder bed with a circulation volume of 40 ml/h·cm²by means of a pump. A direct current is set and regulated at 20 Abetween the anode and the cathode, and electrolysis is carried out for125 h. During the electrolysis, the voltage is in the range of from 2.8V (start)-3.1 V (end), an electrolysis temperature of from 30 to 35° C.being set. Every hour, 210 ml of a sodium hydroxide solution containing100 g/l are added to the electrolysis system, and about 210 ml ofproduct liquor (105 g/l W and 3.5 g/l Re) are taken out via an overflow.After the end of the electrolysis, the remaining cell content is washed,dried and weighed: the quantity of dissolved powder scrap is found to be2845 g, which corresponds to a Faradic current efficiency of 100%. Theenergy consumption per kg of dissolved powder scrap is between 2.46 and2.72 kWh, depending on the voltage.

Although the invention has been described in detail in the foregoing forthe purpose of illustration, it is to be understood that such detail issolely for that purpose and that variations can be made therein by thoseskilled in the art without departing from the spirit and scope of theinvention except as it may be limited by the claims.

1-14. (canceled)
 15. An electrolysis cell, comprising an electricallyinsulating housing and two parallel liquid-permeable electrodes, atleast one electrode having a structure which comprises a supportingpierced plate, an electrode plate provided with perforations, and afilter cloth arranged between the supporting pierced plate and theelectrode plate, the cathode structure being shielded from the cell bymeans of a liquid-permeable separator made of electrically insulatingmaterial, a distributor chamber for the electrolyte being provided onthe cathode side and a collection chamber for the electrolyte beingprovided on the anode side, a recirculation line arranged outside thecell being provided, and means being provided for feeding theelectrolyte back from the collection chamber to the distributionchamber.
 16. The electrolysis cell according to claim 15, wherein theelectrodes being arranged horizontally above one another, the anodeforming the lower electrode, and the cathode being arranged verticallymobile in the housing, wherein the electrode spacing is determined bythe height of the solid-material filling between the electrodes.
 17. Theelectrolysis cell according to claim 15, further comprising means forremoving some of the electrolysis brine and for supplying freshelectrolyte being provided in the distributor channel, the collectionline and/or in the recirculation line.
 18. The electrolysis cellaccording to one of claims 15, further comprising means for drawing offhydrogen being provided in the distributor chamber.
 19. The electrolysiscell according to claim 16, further comprising means for removing someof the electrolysis brine and for supplying fresh electrolyte beingprovided in the distributor channel, the collection line and/or in therecirculation line.
 20. The electrolysis cell according to one of claim16, further comprising means for drawing off hydrogen being provided inthe distributor chamber.
 21. The electrolysis cell according to one ofclaim 17, further comprising means for drawing off hydrogen beingprovided in the distributor chamber.